The present invention is directed to a method for enhancing the formation and recovery of dialkyldihalosilane from alkylhalosilane crude. More particularly, the present invention relates to a direct method for methylchlorosilane manufacture by maintaining an optimum amount of tin and phosphorus in the catalyst.
Prior to the present invention, methylchlorosilanes, hereinafter sometimes designated xe2x80x9cmethylchlorosilanexe2x80x9d for brevity, were made by effecting a reaction between powdered silicon and methyl chloride in the presence of a copper-zinc-tin catalyst, as shown by U.S. Pat. No. 4,500,724, Ward et al., assigned to the same assignee as the present invention. Silicon (Si) is typically combined with copper (Cu) in a range between about 0.5% and about 10% by weight copper relative to silicon, zinc (Zn) in a range between about 0.01 part and about 0.5 part per part copper, and tin (Sn) in a range between about 200 parts per million and about 3,000 parts per million per part of copper.
By the prior art method, there is produced a mixture of alkylhalosilanes and in particular, methylchlorosilanes. As utilized hereinafter, the term xe2x80x9cmethylchlorosilanesxe2x80x9d includes dimethyldichlorosilane (Di), which is the preferred methylchlorosilane, and a variety of other silanes such as tetramethylsilane, trimethylchlorosilane (Mono), methyltrichlorosilane (Tri), silicon tetrachloride, trichlorosilane, methyldichlorosilane (MH), and dimethylchlorosilane (M2H).
In addition to the above methylchlorosilanes, residue is also formed during the production of methylchlorosilane crude. xe2x80x9cCrudexe2x80x9d as used herein refers to the unpurified product mixture of methylchlorosilanes. xe2x80x9cResiduexe2x80x9d as used herein refers to products in the methylchlorosilane crude that have a boiling point greater than about 70xc2x0 C. at atmospheric pressure. Residue typically consists of such materials as disilanes, for example, symmetrical 1,1,2,2-tetrachlorodimethyldisilane, 1,1,2trichlorotrimethyldisilane, disiloxanes, disilmethylenes, and other higher boiling species, for example, trisilanes, trisiloxanes, and trisilmethylenes.
Generally, it is desirable to effect high rates of production in the methylchlorosilane reaction as well as selectively produce dimethyldichlorosilane over the other products. New techniques are constantly being sought to improve the selectivity of the methylchlorosilane reaction as well as increase the yield of the methylchlorosilane products.
The present invention provides a method for promoting the formation of dialkyldihalosilane during direct method alkylhalosilane production, the method comprising effecting reaction between alkyl halide and powdered silicon in the presence of a catalyst comprising copper, zinc, and aluminum and further comprising an amount of phosphorus and tin wherein the amount of phosphorus and tin substantially enhances the selectivity of dialkyldihalosilane formation in comparison to the selectivity of dialkyldihalosilane formation in the presence of the catalyst comprising copper, zinc, aluminum and tin without phosphorus.